The present invention refers to a method of producing a liquid absorbent open-cell polymeric foam material having properties which makes it suitable for use as an absorbent structure in absorbent articles such as diapers, pant diapers, sanitary napkins, incontinence guards, wound dressings, bed protections etc, said method comprising dissolving a polymer in a solvent, adding a surfactant and causing foaming, after which the polymer is crosslinked in the foamed mixture by means of a crosslinking agent.
Absorbent articles of the above mentioned kind are intended to be used for absorption of body liquids such as urine and blood. They usually comprise a liquid pervious topsheet, which during use is intended to be facing the wearer""s body, e g a nonwoven material of spunbond type, a meltblown material, a carded bonded wadding etc. They further have a liquid impervious backsheet, e g a plastic film, a plastic coated nonwoven or a hydrophobic nonwoven, and an absorbent structure arranged between the liquid pervious topsheet material and the liquid impervious backsheet. This absorbent structure may be constructed by several layers such as a liquid acquisition layer, storage layer and distribution layer in order to fulfil the functions which are desired in an absorbent structure: capacity to quickly receive liquid, distribute it in the structure and store it.
As a liquid acquisition layer there is usually used a porous material having a high momentaneous liquid receiving capacity. Such materials are open, bulky structures with large capillaries, for example cellulosic fluff pulp of thermomechanic or chemothermomechanic (CTMP) type, chemically stiffened cellulosic fibers, synthetic fiber structures of different types and porous foam materials etc.
As a storage layer there is usually used cellulosic fluff pulp mixed with so called superabsorbents, which are polymers with the ability to absorb several times their own weight (10 times or more) of body fluids. It is also possible to use an absorbent foam material as a storage layer. As a distribution layer there can be used cellulosic fluff pulp, tissue layers, foam, synthetic fibers and the like having high liquid distribution capacity.
It is also possible to combine two or more of the functions acquisition, storage and distribution in one and the same layer.
It is previously known through U.S. Pat. No. 3,512,450, EP-A-0 293 208 and EP-A-0 804 913 to use a compressed foam material of regenerated cellulose, e g viscose, as an absorbent structure in an absorbent article of the above mentioned kind, e g a sanitary napkin. The article may then be made very thin and still have a high absorption capacity. The compressed viscose foam expands quickly i the z-direction when liquid is absorbed by the material when wetted.
The production of absorbent foams based on polysaccharides by foaming an aqueous solution of a polysaccharide and a surfactant by mechanical agitation or gas supply, and then stabilize the foam by crosslinking with a covalent or ionic crosslinking agent, is previously known through WO 94/00512 and EP-A-0 747 420. The foam may for example be used as a carrier material in medical applications and in wound dressings. Nothing is mentioned about specific absorbent properties.
WO 95/31500 describes the production of absorbent porous foams having a mean pore size below 100 xcexcm. The foam is produced by dissolving a polymer and a crosslinking agent in a solvent, after which a phase separation takes place in a polymer-concentrated phase and a polymer-diluted phase, and where crosslinking occurs in the concentrated phase. The produced foam is said to have an absorbent capacity of at least 2 and preferably at least 10 g/g and be suited as an absorption material in for example diapers.
In EP-B-0 598 833 there is disclosed a foam material intended as an absorbent structure of the above stated kind. The foam material has a specified pore volume, specific surface area and ability to resume its volume after compression. The foam is a so called xe2x80x9cHIPExe2x80x9d-foam (high internal phase emulsion), which means that the foam is produced by polymerization of a water-in-oil emulsion. The solid phase in the foam creates a capillary system, which receives, distributes and stores liquid. There is no indication about the liquid storage capacity of the foam measured by CRC (centrifuge retention capacity), which is a measure of the capacity of the foam to firmly bind liquid, so called gel liquid, in its solid phase by swelling the cell walls.
The object of the invention is to provide a method of producing a foam material suited to be used as an absorbent structure in an absorbent article of the above mentioned kind and which has multifunctional properties in such a way, that it at the same time fulfils the function of a liquid acquisition layer, a storage layer and a distribution layer, namely the capacity to quickly receive liquid, distribute it in the structure and store it. The method should further be adapted for a large number of polymeric materials, including polymers based on renewable raw materials such as polysaccharides and polypeptides.
This has according to the invention been provided by dissolving a polymer in a solvent, adding a surfactant and cause foaming, adding a crosslinking agent to the foamed mixture, after which the temperature of the foam thus formed is lowered below the freezing point of the solvent and letting the crosslinking reaction continue during the freezing step, and that after that the main part of the solvent is removed from the foam material.
By the freezing step the foam is given unique properties concerning liquid absorption and obtains a firm porous structure at the same time as it is soft and flexible. It is assumed that the freezing step influences the crosslinking in a positive way, which in turn influences the structure and shape stability of the foam. The foam may also be compressed to a high density and then be able to swell and expand upon contact with liquid.
The polymer is preferably a polysaccharide or a polypeptide.
The solvent is preferably water.
According to one embodiment fibers can be added to the polymer solution, preferably hydrophilic fibers such as for example cellulosic fibers. It can sometimes be desired to produce absorbent products having an anatomic three-dimensional shape, which with the foam according to the present invention can be provided by after foaming and before freezing applying the foam in a mould, and keeping the foam in the mould during the freezing step.